This invention relates generally to magnetic resonance imaging (MRI), and more particularly the invention relates to reduced imaging time in projection reconstruction using magnetic resonance data collected in k-space.
Nuclear magnetic resonance (NMR) imaging, also called magnetic resonance imaging (MRI), is a non-destructive method for the analysis of materials and represents a new approach to medical imaging. It is completely non-invasive and does not involve ionizing radiation. In very general terms, nuclear magnetic moments are excited at specific spin precession frequencies which are proportional to the local magnetic field. The radio-frequency signals resulting from the precession of these spins are received using pickup coils. By manipulating the magnetic fields, an array of signals is provided representing different regions of the volume. These are combined to produce a volumetric image of the nuclear spin density of the body.
A descriptive series of papers on NMR appeared in the June 1980 issue of the IEEE Transactions on Nuclear Science Vol. NS-27, pp. 1220-1225. The basic concepts are described in the lead article, "Introduction to the Principles of NMR," by W. V. House, pp. 1220-1226, which employ computed tomography reconstruction concepts for reconstructing cross-sectional images. A number of two- and three-dimensional imaging methods are described. Medical applications of NMR are discussed by Pykett in "NMR Imaging in Medicine," Scientific American, May 1982, pp. 78-88, and by Mansfield and Morris, NMR Imaging in Biomedicine, Academic Press, 1982.
Briefly, a strong static magnetic field is employed to line up atoms whose nuclei have an odd number of protons and/or neutrons, that is, have spin angular momentum and a magnetic dipole moment. A second RF magnetic field, applied as a single pulse transverse to the first, is then used to pump energy into these nuclei, flipping them over, for example to 90.degree. or 180.degree.. After excitation the nuclei gradually return to alignment with the static field and give up the energy in the form of weak but detectable free induction decay (FID). These FID signals are used by a computer to produce image.
The excitation frequency, and the FID frequency, is defined by the Larmor relationship which states that the angular frequency, .omega..sub.0, of the precession of the nuclei is the product of the magnetic field, B.sub.0, and the so-called magnetogyric ratio, .gamma., a fundamental physical constant for each nuclear species: EQU .omega..sub.0 =B.sub.0 .multidot..gamma.
Accordingly, by superimposing a linear gradient field, B.sub.z =z.multidot.G.sub.z, on the static uniform field, B.sub.0, which defines the Z axis, for example, nuclei in a selected X-Y plane can be excited by proper choice of the frequency spectrum of the transverse excitation field applied along the X or Y axis. Similarly, a gradient field can be applied in the X-Y plane during detection of the FID signals to spatially localize the FID signals in the plane. The angle of nuclei spin flip in response to an RF pulse excitation is proportional to the integral of the pulse over time.
Projection reconstruction in MR imaging has become of interest recently for the imaging of short-T.sub.2 species, the lung parenchyma, and flow. For the form of projection reconstruction in which data is acquired along radial lines with Free Induction Decay (FID) readouts, no dephasing or phase encoding period is required prior to the data acquisition. The effective echo time can then be very short, as short as 250 .mu.s on a General Electric whole body scanner, which is useful for the above applications because the image can be acquired prior to any substantial T.sub.2 decay to signal dephasing due to susceptibility or complex flow. Projection reconstruction sequences have also been found useful for the reduction of motion artifacts and displacement artifact that arises in flow imaging from different phase-encoding and readout times.
U.S. Pat. No. 5,025,216 discloses a method of magnetic resonance imaging of short T.sub.2 species using slice-selective excitations based on k-space excitation. While the conventional model of magnetic resonance imaging is based on the correspondence between the spectrum of a transient response (the FID signal) and projections of an unknown spatial distribution, a k-trajectory model views the FID as the spatial-frequency distribution which is the Fourier transform of the unknown spatial distribution to be imaged. See Tweig "The k-trajectory formulation of the NMR imaging process with applications in analysis and synthesis of imaging methods," Medical Physics 10(5) pp. 610-621 September/October 1983. The k-trajectory is the path in spatial-frequency space over which the sampling process occurs, and it is determined by the gradient fields applied during the FID.
The radial line k-space collection strategy is, however, a very inefficient way to cover k space. With the traditional 2-D Fourier transform or spin-warp coverage patterns, a line is typically collected from edge to edge in k space. For the radial line method, data actuation begins at the center k space and proceeds outward, so only 1/2 of one line in k space is collected with each excitation. This combined with non-constant sampling density in k space causes this inefficiency. Therefore, for a given resolution and field-of-view, the required number of excitations and consequently, the imaging times are increased by a factor of .pi. over spin-warp methods.
The present invention is directed to a k-space reconstruction method for projection reconstruction of magnetic response images allowing reduced imaging time.